Acoustic matrix filters and radios using acoustic matrix filters

ABSTRACT

There are disclosed acoustic filters and radios incorporating the acoustic filters. A filter includes a first filter port, a second filter port, and n sub-filters, where n is an integer greater than one. Each sub-filter has a first sub-filter port connected to the first filter port and a second sub-filter port connected to the second filter port. A first acoustic resonator is connected from the first filter port to ground, and a second acoustic resonator is connected from the second filter port to ground. The first and second acoustic resonators are configured to create respective transmission zeros adjacent to a lower edge of a passband of the filter.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application63/087,789, filed Oct. 5, 2020, entitled MATRIX XBAR FILTER.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz. Matrix XBARfilters are also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR).

FIG. 2A is an equivalent circuit model of an acoustic resonator.

FIG. 2B is a graph of the admittance of an ideal acoustic resonator.

FIG. 2C is a circuit symbol for an acoustic resonator.

FIG. 3A is a schematic diagram of a matrix filter using acousticresonators.

FIG. 3B is a schematic diagram of a sub-filter of FIG. 3A.

FIG. 4 is a graph of the performance of an embodiment of the filter ofFIG. 3A.

FIG. 5 is a graph of input-output transfer functions of the individualsub-filters of the embodiment of FIG. 4.

FIG. 6 is a schematic diagram of a matrix diplexer using acousticresonators.

FIG. 7 is a graph of input-output transfer functions of an embodiment ofthe diplexer of FIG. 6.

FIG. 8 is a schematic diagram of a matrix multiplexer using acousticresonators.

FIG. 9 is a graph of input-output transfer functions of an embodiment ofthe multiplexer of FIG. 8.

FIG. 10A is a schematic diagram of a reconfigurable matrix filter usingacoustic resonators.

FIG. 10B is a schematic diagram of a sub-filter and switch module ofFIG. 10A.

FIG. 11 is a graph of input-output transfer functions of twoconfigurations of an embodiment of the reconfigurable matrix filter ofFIG. 10A.

FIG. 12 is a block diagram of a time division duplex radio using amatrix filter.

FIG. 13 is a block diagram of a frequency division duplex radio using amatrix filter.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR) 100. XBAR resonators such as theresonator 100 may be used in a variety of RF filters includingband-reject filters, band-pass filters, duplexers, and multiplexers.XBARs are particularly suited for use in filters for communicationsbands with frequencies above 3 GHz. The matrix XBAR filters described inthis patent are also suited for frequencies above 1 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. The piezoelectric platemay be Z-cut (which is to say the Z axis is normal to the front and backsurfaces 112, 114), rotated Z-cut, or rotated YX cut. XBARs may befabricated on piezoelectric plates with other crystallographicorientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. The primary acoustic mode of an XBAR is abulk shear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate which spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may have more or fewer thanfour sides, which may be straight or curved.

The detailed cross-section view (Detail C) shows two IDT fingers 136 a,136 b on the surface of the piezoelectric plate 110. The dimension p isthe “pitch” of the IDT and the dimension w is the width or “mark” of theIDT fingers. A dielectric layer 150 may be formed between and optionallyover (see IDT finger 136 a) the IDT fingers. The dielectric layer 150may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. The dielectric layer 150 may be formed of multiplelayers of two or more materials. The IDT fingers 136 a and 136 b may bealuminum, copper, beryllium, gold, tungsten, molybdenum, alloys andcombinations thereof, or some other conductive material. Thin (relativeto the total thickness of the conductors) layers of other metals, suchas chromium or titanium, may be formed under and/or over and/or aslayers within the fingers to improve adhesion between the fingers andthe piezoelectric plate 110 and/or to passivate or encapsulate thefingers and/or to improve power handling. The busbars of the IDT 130 maybe made of the same or different materials as the fingers.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds ofparallel fingers in the IDT 110. Similarly, the thickness of the fingersin the cross-sectional views is greatly exaggerated.

An XBAR based on shear acoustic wave resonances can achieve betterperformance than current state-of-the art surface acoustic wave (SAW),film-bulk-acoustic-resonators (FBAR), and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices. In particular, the piezoelectriccoupling for shear wave XBAR resonances can be high (>20%) compared toother acoustic resonators. High piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters ofvarious types with appreciable bandwidth.

The basic behavior of acoustic resonators, including XBARs, is commonlydescribed using the Butterworth Van Dyke (BVD) circuit model as shown inFIG. 2A. The BVD circuit model consists of a motional arm and a staticarm. The motional arm includes a motional inductance L_(m), a motionalcapacitance C_(m), and a resistance R_(m). The static arm includes astatic capacitance C₀ and a resistance R₀. While the BVD model does notfully describe the behavior of an acoustic resonator, it does a good jobof modeling the two primary resonances that are used to design band-passfilters, duplexers, and multiplexers (multiplexers are filters with morethan 2 input or output ports with multiple passbands).

The first primary resonance of the BVD model is the motional resonancecaused by the series combination of the motional inductance L_(m) andthe motional capacitance C_(m). The second primary resonance of the BVDmodel is the anti-resonance caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the motional resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$

The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$

where γ=C₀/C_(m) is dependent on the resonator structure and the typeand the orientation of the crystalline axes of the piezoelectricmaterial.

FIG. 2B is a graph 200 of the magnitude of admittance of a theoreticallossless acoustic resonator. The acoustic resonator has a resonance 212at a resonance frequency where the admittance of the resonatorapproaches infinity. The resonance is due to the series combination ofthe motional inductance L_(m) and the motional capacitance C_(m) in theBVD model of FIG. 2A. The acoustic resonator also exhibits ananti-resonance 214 where the admittance of the resonator approacheszero. The anti-resonance is caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$

The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$

In over-simplified terms, the lossless acoustic resonator can beconsidered a short circuit at the resonance frequency 212 and an opencircuit at the anti-resonance frequency 214. The resonance andanti-resonance frequencies in FIG. 2B are representative, and anacoustic resonator may be designed for other frequencies.

FIG. 2C shows the circuit symbol for an acoustic resonator such as anXBAR. This symbol will be used to designate each acoustic resonator inschematic diagrams of filters in subsequent figures.

FIG. 3A is a schematic diagram of a matrix filter 300 using acousticresonators. The matrix filter 300 includes an array 310 of n sub-filters320-1, 320-2, 320-n connected in parallel between a first filter port(FP1) and a second filter port (FP2), where n is an integer greater thanone. The sub-filters 320-1, 320-2, 320-n have contiguous passbands suchthat the bandwidth of the matrix filter 300 is equal to the sum of thebandwidths of the constituent sub-filters. In the subsequent examples inthis patent n=3. n can be less than or greater than 3 as necessary toprovide the desired bandwidth for the matrix filter 300.

The array 310 of sub-filters is terminated at both ends by acousticresonators XL1, XL2, XH1, and XH2, which are preferably but notnecessarily XBARs. The acoustic resonators XL1, XL2, XH1, and XH2 create“transmission zeros” at their respective resonance frequencies. A“transmission zero” is a frequency where the input-output transferfunction of the filter is very low (and would be zero if the acousticresonators XL1, XL2, XH1, and XH2 were lossless). Typically, theresonance frequencies of XL1 and XL2 are equal, and the resonancefrequencies of XH1 and XH2 are equal. The resonant frequencies of theacoustic resonators XL1, XL2 are selected to provide transmission zerosadjacent to the lower edge of the filter passband. The acousticresonators XL1 and XL2 also act as shunt inductances to help match theimpedance at the ports of the filter to a desired impedance value. Inthe subsequent examples in this patent, the impedance at all ports ofthe filters is matched to 50 ohms. The resonant frequencies of acousticresonators XH1, XH2 are selected to provide transmission zeros at orabove the higher edge of the filter passband. Acoustic resonators XH1and XH2 may not be required in all matrix filters.

FIG. 3B is a schematic diagram of a sub-filter 350 suitable forsub-filters 320-1, 320-2, and 320-n. The sub-filter 350 includes threeacoustic resonators X1, X2, X3 connected in series between a firstsub-filter port (SP1) and a second sub-filter port (SP2). The acousticresonators X1, X2, X3 are preferably but not necessarily XBARs. Thesub-filter 350 includes two coupling capacitors C1, C2, each of which isconnected between ground and a respective node between two of theacoustic resonators. The inclusion of three acoustic resonators in thesub-filter 350 is exemplary. A sub-filter may have m acousticresonators, where m is an integer greater than one. A sub-filter with macoustic resonators includes m−1 coupling capacitors. The in acousticresonators of a sub-filter are connected in series between the two portsSP1 and SP2 of a sub-filter and each of the m−1 coupling capacitors isconnected between ground and a node between a respective pair ofacoustic resonators from the in acoustic resonators.

Compared to other types of acoustic resonators, XBARs have very highelectromechanical coupling (which results in a large difference betweenthe resonance and anti-resonance frequencies), but low capacitance perunit area. The matrix filter architecture, as shown in FIG. 3A and FIG.3B, takes advantage of the high electromechanical coupling of XBARswithout requiring high resonator capacitance.

FIG. 4 is a graph 400 of the performance of an exemplary embodiment of amatrix filter implemented using XBARs for all of the acousticresonators. Specifically, the solid line 410 is a plot of S21, the FP1to FP2 transfer function, of the filter as a function of frequency. Thedashed line 420 is a plot of S11, the return loss at FP1, as a functionof frequency. Since the exemplary filter is symmetrical, the solid line410 and the dashed line 420 are also plots of S12 and S22. respectively.The matrix filter includes 3 sub-filters, with each sub-filter includingthree XBARs, as shown in FIG. 3A and FIG. 3B. In this example and allsubsequent examples, filter performance was determined by simulating thefilter using BVD models (FIG. 2A) for the XBARs.

The characteristics of the components of the matrix filter are providedin TABLE 1. Each XBAR is defined by its resonance frequency Fr andstatic capacitance C0. The Q of each XBAR is assumed to be 1000. γ isassumed to be 2.5, which is representative of lithium niobate XBARs. Thesame assumptions and component values are used in all subsequentexamples in this patent.

TABLE 1 X1 X2 X3 Fr C0 Fr C0 Fr C0 C1 C2 (MHz) (pF) (MHz) (pF) (MHz)(pF) (pF) (pF) Sub- 1949 0.100 1933 0.100 1949 0.100 1.49 1.49 filter 1Sub- 2011 0.077 1991 0.077 2011 0.077 1.31 1.31 filter 2 Sub- 2067 0.0702049 0.070 2067 0.070 1.33 1.33 filter 3 XL1, 1915 0.168 XL2 XH1, 21400.081 XH2

The exemplary matrix filter is symmetrical in that the impedances atPort 1 and Port 2 are both equal to 50 ohms. The internal circuitry ofthe filter is also symmetrical, with XBARs X1 and X3 within eachsub-filter being the same, XL1 and XL2 being the same, and XH1 and XH2being the same. Matrix filters may be designed to have significantlydifferent impedances at Port 1 and Port 2, in which event the internalcircuitry will not be symmetrical.

FIG. 5 is a graph of the characteristics of the elements within theexemplary matrix filter whose performance was shown in FIG. 4.Specifically, solid line 510, dashed line 520, and dotted line 530 areplots of the magnitude of the input-output transfer functions forsub-filter 1, sub-filter 2, and sub-filter 3, respectively.

The input-output transfer function of the exemplary filter, as shown inFIG. 4, is the vector sum of the input-output transfer functions of thethree sub-filters. To this end, the input-output transfer functions ofsub-filter 1 and sub-filter 2 cross at a frequency where (a) S21 of bothfilters are substantially equal to −3 dB and (b) the phases of theinput-output transfer functions of both filters are substantially equal.In this context, “substantially equal” means sufficiently equal to notcause objectionable variations in the insertion loss of the matrixfilter within the filter passband. The quantitative value of“substantially equal” may be different for different filterapplications. Similar requirements apply to sub-filter 2 and sub-filter3. In matrix filters with more than three sub-filters, similarrequirements apply to every adjacent (in frequency) pair of sub-filters.

The vertical dot-dash lines identify the resonance frequencies of theXBARs within the exemplary matrix filter. The line labeled “XL”identifies the resonance frequency of the resonators XL1 and XL2, whichis adjacent to the lower edge of the filter passband. Similarly, theline labeled “XH” identifies the resonance frequency of the resonatorsXH1 and XH2, which is adjacent to the upper edge of the filter passband.The two lines labeled “SF1” identify the resonance frequencies of theXBARs within sub-filter 1 in isolation. Note that both of the resonancefrequencies are lower than the center of the passband. This is becausethe resonance frequency of a resonator and a capacitor in series ishigher that the resonance frequency of the resonator in isolation.Similarly, the two lines labeled “SF2” identify the resonancefrequencies of the XBARs within sub-filter 2 and the two lines labeled“SF3” identify the resonance frequencies of the XBARs within sub-filter3.

FIG. 6 is a schematic diagram of a matrix filter 600 configured as adiplexer. The matrix filter 600 includes an array 610 of threesub-filters 620-1, 620-2, 620-n. Sub-filter 1 620-1 is connected betweena first filter port (FP1) and a second filter port (FP2). Sub-filter 2620-2 and sub-filter 3 620-3 are connected in parallel between FP1 and athird filter port (FP3). FP1 is the common port of the diplexer and FP2and FP3 are the branch ports. The array 610 of sub-filters is terminatedat both ends by XBARs XL and XH as previously described.

FIG. 7 is a graph 700 of the performance of an example of the matrixfilter diplexer 600 of FIG. 6. In this example, XL, XH, and the threesub-filters are the same as the corresponding elements of the matrixfilter 300 of FIG. 3A. In FIG. 7, the solid line 710 is a plot of S21,the FP1 to FP2 transfer function, as a function of frequency. The dashedline 720 is a plot of S31, the FP1 to FP3 transfer function, as afunction of frequency. Since the exemplary filter is symmetrical, thesolid line 710 and the dashed line 720 are also plots of S12 and S13,respectively. The matrix filter 600 is exemplary. In most applications,a diplexer will have the same number (two, three or more) sub-filters inparallel between the common port and the two branch ports.

FP1 may be considered the common port of the matrix filter diplexer 600.FP2 may be considered the “low band” port and FP3 may be considered the“high band” port. When the matrix filter diplexer is used in a frequencydivision duplex radio, one of FP2 and FP3 may be the receive port of thediplexer and the other of FP2 and FP3 may be the transmit port of thediplexer depending on the frequencies allocated for reception andtransmission.

FIG. 8 is a schematic diagram of a matrix filter 800 configured as amultiplexer. The matrix filter 800 includes an array 810 of threesub-filters 820-1, 820-2, 820-n. Sub-filter 1 820-1 is connected betweena first filter port (FP1) and a second filter port (FP2). Sub-filter 2820-2 is connected between FP1 and a third filter port (FP3). Sub-filter3 820-3 is connected between FP1 and a fourth filter port (FP4). Thearray 810 of sub-filters is terminated at both ends by XBARs XL and XHas previously described. FP1 is the common port of the multiplexer andFP2, FP3, and FP4 are branch ports of the multiplexer. A multiplexer mayhave more than three branch ports.

FIG. 9 is a graph 900 of the performance of an example of the matrixfilter multiplexer 800 of FIG. 8. In this example, XL, XH, and the threesub-filters are the same as the corresponding elements of the matrixfilter 300 of FIG. 3A. In FIG. 9, the solid line 910 is a plot of S21,the FP1 to FP2 transfer function, as a function of frequency. The dashedline 920 is a plot of S31, the FP1 to FP3 transfer function, as afunction of frequency. The dotted line 930 is a plot of S41, the FP1 toFP4 transfer function, as a function of frequency. Since the exemplaryfilter is symmetrical, the solid line 910, the dashed line 920, and thedotted line 930 are also plots of S12, S13 and S14, respectively.

FIG. 10A is a schematic diagram of a reconfigurable matrix filter 1000using XBARs. The reconfigurable matrix filter 1000 includes an array1010 of n sub-filter/switch circuits 1020-1, 1020-2, 1020-n connected inparallel between a first filter port (FP1) and a second filter port(FP2), where n is an integer greater than one. In a subsequent example,n=3. n can be greater than 3 as necessary to provide the desiredbandwidth for the reconfigurable matrix filter 1000. Eachsub-filter/switch circuit functions as a bandpass filter that can beselectively enabled (i.e. connected between FP1 and FP2) or disabled(i.e. not connected between FP1 and FP2). The array 1010 ofsub-filter/switch circuits is terminated at both ends by XBARs XL and XHas previously described.

The sub-filter/switch circuits 1020-1, 1020-2, 1020-n have contiguouspassbands such that the bandwidth of the matrix filter 1000, when allsub-filter/switch modules are enabled, is equal to the sum of thebandwidths of the constituent sub-filters. One or more of thesub-filter/switch circuits can be disabled to tailor the matrix filterbandwidth or to insert notches or stop bands within the overallpassband.

FIG. 10B is a schematic diagram of a sub-filter/switch circuit 1050suitable for sub-filter/switch circuits 1020-1, 1020-2, and 1020-n inFIG. 10A. The sub-filter/switch circuit 1050 includes three acousticresonators X1, X2, X3 in series between a first sub-filter port (SP1)and a second sub-filter port (SP2), and coupling capacitors C1, C2connected from the junctions between adjacent acoustic resonators toground. The inclusion of three acoustic resonators in thesub-filter/switch circuit 1050 is exemplary, and a sub-filter/switchcircuit may have more than three acoustic resonators. When asub-filter/switch circuit includes more than three acoustic resonators,the number of coupling capacitors will be one less than the number ofacoustic resonators. The acoustic resonators X1, X2, X3 are preferablybut not necessarily XBARs.

The sub-filter/switch circuit 1050 includes a switch SW in series withacoustic resonator X2. When the switch SW is closed, thesub-filter/switch circuit operates as a sub-filter suitable for use inany of the prior examples. When the switch SW is open, thesub-filter/switch circuit presents the proper impedance to SP1 and SP2but has the input-output transfer function of an open circuit. When asub-filter/switch circuit includes more than three acoustic resonators,the switch may be in series with any of the acoustic resonators otherthan the two acoustic resonators connected to the two sub-filter ports.In other words, the switch can be in series with any of the “middleacoustic resonators” in the middle of the string of resonators, but notthe two “end acoustic resonators” at the ends of the string.

FIG. 11 is a graph 1100 of the performance of an example of thereconfigurable matrix filter diplexer 1000 of FIG. 10. In this example,XL, XH, and the components within the three sub-filter/switch circuitsare the same as the corresponding elements of the matrix filter 300 ofFIG. 3A. In FIG. 11, the solid line 1130 is a plot of S21, the port 1 toport 2 transfer function, of the filter as a function of frequency whensub-filter/switch circuit 1 is enabled and sub-filter/switch circuit 2and 3 are disabled. The dashed line 1110 is a plot of S21 as a functionof frequency when sub-filter/switch circuit 3 is enabled andsub-filter/switch circuit 1 and 2 are disabled. The sum of the twocurves 1110 and 1130, not shown but easily envisioned, is the port 1 toport 2 transfer function as a function of frequency whensub-filter/switch circuits 1 and 3 are enabled and sub-filter/switchcircuit 2 is disabled. A total of eight different filter configurationsare possible by enabling various combinations of the threesub-filter/switch circuits.

FIG. 12 is a schematic block diagram of a time division duplex (TDD)radio 1200. A TDD radio transmits and receives in the same frequencychannel within a designated communications band. The radio 1200 includesa matrix bandpass filter 1210 having a first filter port FP1 configuredto connect to an antenna 1205 and a second filter port FP2 coupled to atransmit/receive (T/R) switch 1215. The T/R switch 1215 connects thesecond port of the matrix bandpass filter 1210 to either the output of atransmitter 1220 or the input of a receiver 1225. The T/R switch 1215,the transmitter 1220, and the receiver 1225 are supervised by aprocessor 1230 performing a media access control function. Specifically,the processor 1230 controls the operation of the T/R switch 1215 and,when the matrix bandpass filter 1210 is reconfigurable, the processor1230 may control the operation of switches within the bandpass filter.The antenna 1205 may be a part of the radio 1200 or external to theradio 1200.

The radio 1200 is configured for operation in the designatedcommunications band. The matrix bandpass filter 1210 has a pass bandthat encompasses the designated communications band and one or more stopbands to block designated frequencies outside of the designatedcommunications band. Preferably, the bandpass filter 1210 has low lossin its pass band and high rejection in its stop band(s). Further, thebandpass filter 1210 must be compatible with TDD operation, which is tosay stable and reliable while passing the RF power generated by thetransmitter 1220. The matrix bandpass filter 1210 may be the matrixfilter 300 of FIG. 3A or the reconfigurable matrix filter 1000 of FIG.10A implemented using acoustic resonators which may be XBARs.

The matrix bandpass filter 1210 may be a reconfigurable matrix filter asshown in FIG. 10A. The use of a reconfigurable filter would allow thebandwidth of the filter to be reduced to a single channel or group ofchannels within the designated communications band. This may reduceinterference from and with other radios communicating in the samecommunications band. When the matrix bandpass filter 1210 isreconfigurable, the switches within the sub-filters may be controlled bythe process 1230.

FIG. 13 is a schematic block diagram of a frequency division duplex(FDD) radio 1300. An FDD radio transmits and receives in differentfrequency ranges with a defined communications band. The transmit andreceive frequency ranges are typically, but not necessarily, adjacent.The radio 1300 includes an antenna 1305, a matrix filter diplexer 1310having a common filter port FP1 configured to connect to an antenna1305, a transmit filter port FP3 coupled to the output of a transmitter1320, and a receive filter port FP2 coupled to the input of a receiver1325.

The radio 1300 is configured for operation in the designatedcommunications band. The matrix filter diplexer 1310 includes a receivefilter coupled between FP1 and FP2 and a transmit filter coupled betweenFP1 and FP3. The receive filter may include one or more receivesub-filters. The transmit filter may include one or more transmitsub-filters. The transmit filter must be compatible with the RF powergenerated by the transmitter 1320. The matrix filter diplexer 1310 maybe implemented using acoustic resonators which may be XBARs.

The matrix filter diplexer 1310 may be similar to the matrix diplexer600 of FIG. 6 with an equal number of sub-filters in the transmit andreceive filters. The common filter port of the matrix filter diplexer1310 may be FP1 of the matrix diplexer 600 may be port 1 of the diplexer600. The transmit port TP may be either of FP2 or FP3, and the receiveport RP may be the other of FP2 and FP3.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A filter, comprising: a first filter port and a secondfilter port; n sub-filters, where n is an integer greater than one, eachof the n sub-filters having a first sub-filter port connected to thefirst filter port and a second sub-filter port connected to the secondfilter port; a first acoustic resonator connected between the firstfilter port and ground; and a second acoustic resonator connectedbetween the second filter port and ground, wherein the first and secondacoustic resonators are configured to create respective transmissionzeros adjacent to a lower edge of a passband of the filter.
 2. Thefilter of claim 1, wherein the passband of the filter is equal to a sumof passbands of the n sub-filters.
 3. The filter of claim 1 wherein thefirst and second acoustic resonators are transversely-excited film bulkacoustic resonators (XBARs).
 4. The filter of claim 1, furthercomprising: a third acoustic resonator connected between the firstfilter port and ground; and a fourth acoustic resonator connectedbetween the second filter port and ground, wherein the third and fourthacoustic resonators are configured to create transmission zeros adjacentto an upper edge of the passband of the filter.
 5. The filter of claim 4wherein the third and fourth acoustic resonators aretransversely-excited film bulk acoustic resonators (XBARs).
 6. Thefilter of claim 1, each of the n sub-filters comprising: m acousticresonators connected in series between the first sub-filter port and thesecond sub-filter port, where m is an integer greater than one; and m−1capacitors, each capacitor connected between ground and a node between arespective pair of acoustic resonators from the in acoustic resonators.7. The filter of claim 6 wherein the in acoustic resonators in each ofthe n sub-filters are transversely-excited film bulk acoustic resonators(XBARs).
 8. The filter of claim 6, wherein the in acoustic resonators ofeach of the n sub-filters include a first end acoustic resonatorconnected to the first sub-filter port, a second end acoustic resonatorconnected to the second sub-filter port, and one or more middle acousticresonators connected between the first and second end acousticresonator, and each of the n sub-filters further comprises a switch inseries with one of the one or more middle resonators.
 9. A time divisionduplex (TDD) radio, comprising: a transmitter; a receiver; a filter,comprising: a first filter port configured for connecting to an antennaand a second filter port; n sub-filters, where n is an integer greaterthan one, each sub-filter having a first sub-filter port connected tothe first filter port and a second sub-filter port connected to thesecond filter port; a first acoustic resonator connected between thefirst filter port and ground; and a second acoustic resonator connectedbetween the second filter port and ground, wherein the first and secondacoustic resonators are configured to create respective transmissionzeros adjacent to a lower edge of a passband of the filter; and atransmit/receiver switch to selectively connect the second filter portto one of an output of the transmitter and an input of the receiver. 10.The radio of claim 9, wherein the passband of the filter is equal to asum of passbands of the n sub-filters.
 11. The radio of claim 9, whereinthe first and second acoustic resonators are transversely-excited filmbulk acoustic resonators (XBARs).
 12. The radio of claim 9, furthercomprising: a third acoustic resonator connected between the firstfilter port and ground; and a fourth acoustic resonator connectedbetween the second filter port and ground, wherein the third and fourthacoustic resonators are configured to create transmission zeros adjacentto an upper edge of the passband of the filter.
 13. The radio of claim12, wherein the third and fourth acoustic resonators aretransversely-excited film bulk acoustic resonators (XBARs).
 14. Theradio of claim 9, each of the n sub-filters comprising: m acousticresonators connected in series between the first sub-filter port and thesecond sub-filter port, where m is an integer greater than one; and m−1capacitors, each capacitor connected between ground and a node between arespective pair of acoustic resonators from the in acoustic resonators.15. The radio of claim 14, wherein the in acoustic resonators in each ofthe n sub-filters are transversely-excited film bulk acoustic resonators(XBARs).
 16. The radio of claim 14, wherein the in acoustic resonatorsof each of the n sub-filters include a first end acoustic resonatorconnected to the first sub-filter port, a second end acoustic resonatorconnected to the second sub-filter port, and one or more middle acousticresonators connected between the first and second end acousticresonator, and each of the n sub-filters further comprises a switch inseries with one of the one or more middle resonators.
 17. The radio ofclaim 16, further comprising: a processor to control thetransmit/receive switch and the switches within each of the nsub-filters.
 18. The Radio of claim 9, further comprising the antenna.